1. Field of the Invention
This invention relates generally to sheet-fed printing machines, and more particularly to a sheet feed mechanism.
2. Description of the Background Art
In a high speed sheet fed press having a number of press units for multi-color printing one of the most troublesome problems is to feed a sheet into position accurately and with proper timing at each of the press units thereby to ensure exact register of successively printed impressions. German patent specification No. 1,186,473 teaches use of an air blast in a feed table directed substantially parallel to the table with the intention to hold down and at the same time propel the successive sheets by combined blast-suction effect. A disadvantage of the disclosed arrangement is that it cannot be satisfactorily adapted to different conditions or different materials as, for example, the thickness, stiffness and surface texture of the sheets being handled, or the amount and distribution of moisture in the sheet which may cause waviness at the sheet edges. Finally, a smooth feed plate is used which is found to produce an overly thick cushion of air under the sheet which may produce flutter and whistling of the sheet and even loss of control when handling lightweight grades of paper.
Use of jets of air in a feed table is also taught in U.S. Pat. No. 3,370,847. While this patent does show adjustability of the angle of discharge, the jets are concentrated and spaced along the width dimension of the sheet, and therefore engage the sheet only at widely separated intervals rather than uniformly, resulting in point-to-point variations along the sheet making it difficult to adjust for sheets of light and heavy gauge. Since a smooth feed table is used the arrangement exhibits the same problem of an overly thick cushion of air noted in connection with the above-mentioned German patent. Moreover, because of use of individual nozzles the device consists of numerous parts and is inherently of flimsy construction giving rise to maintenance problems.